If I interpret it correctly, it has an open-source core, but for actual playback it uses fmod, which is not exactly open. This will also make stuff like dsp plugins rather difficult. I prefer foobar's open sdk and plugins over this open core but 3rd-party closed sound library, but ymmv.

I tried both and I was quite impressed with VUPlayer. Foobar, of course, has much more functionality, but as far as as being a non-skinned player VU has some mature features. Musik didn't impress me as much, especially as most of my music is encoded using AAC, which meant I was only able to play about 10 songs on it. My personal attitude toward software (and most other things) is "to each his own," so if someone likes a particular product that doesn't really do it for me, fine. Not being a fan of skinned applications, I'm just happy to see there are other non-skinned players being developed.